Atari's 40th anniversary is this week, and to celebrate they're offering their catalog of classic games on iOS for free. Normally the 100-strong collection goes for $9.99, and even then, most of the titles still required in-app purchases. The version available now offers all of 'em for free, so long as you keep the arcade app installed, but it will be reverting back to its original pricing sometime tonight, so old-school gamers will want to hop on this deal soon. The list of included games include all of the big name titles:

If you have already downloaded the Atari app before, as I had, update it.
It shouldn't require any more effort than that. From then on, whenever I selected a game, it would begin downloading it.
I hope that helps.